Field Weights - Charge
A charge force field can work like honey or a magnet, as particles move close to it, they slow down or stick to it depending on the settings, with two opposing fields (one on the particle system) you can get a magnetic result that draws the particles to the force field.

Note that gravity overpowers a charge, so you should set gravity to 0.0 to see the effect.

The charge setting in the particles field weights defines whether a particle sticks to the charge field, as you increase it particles passing through the charge field will start to stick.

By also having a charge field on the particle emitter you can use positive and negative values to have the particles drawn to or repelled from the field, like magnets.